Magnetic recording device

ABSTRACT

In an optical recording and magnetic-head reproducing system, the reproduced output is prevented from starting to decrease, upon the length of record bits getting shorter, in the range of the bit length larger than reproducing resolution of a magnetic reproducing head. When data are recorded magneto-optically on a magnetic recording medium, the radius of curvature of arcs constituting the boundary of recorded magnetic domains is made as large as possible so that the shape of the recorded spot may be substantially rectangular. As a result, it becomes possible to improve the efficiency of magnetic reproduction, and high-output as well as high-SN-ratio reproduction at high linear recording density can be achieved.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a magnetic recording device which has ameans to record signals as magnetic data optically and reproduce thedata with a magnetic head. More specifically, the present inventionrelates to a magnetic recording device with high resolution and anexcellent SN (signal-to-noise) ratio even in high-density recording.

2. Description of the Related Art

With the rapidly increasing amount of digital information spreading inour society in recent years, the storage capacities of storage mediasuch as magnetic disks, floppy disks, and magnetic tapes and those ofoptical memory devices have been increasing quite rapidly. The recordingdensities of magnetic-disc devices in particular have been increasing by40-60% per annum, and some of them have as high an areal recordingdensity as 4 Gb/in². The densification of magnetic recording devices hasbeen supported by such technological innovation as increases in theresolution and the SN ratios of magnetic disks and the sensitivity ofmagnetic heads. In the case of magnetic disks, their resolution and SNratios have been enhanced mainly by reducing the size of particles whichform their magnetic thin films. However, the approach has a physicallimit; i.e., thermal instability. In other words, magnetic particleshave to be reduced in size to reduce the noise of the thin films ofmagnetic recording media, whereas reducing the particle size beyond acertain level causes thermal instability which, in turn, causes therecorded data of the media to vanish. Thus, there is a trade-offrelation between the SN ratio and the thermal stability. To solve thisproblem, the technology of perpendicular magnetic recording andthermomagnetic recording is being developed. The advantage of theperpendicular magnetic recording is that the thin film of aperpendicular-recording disk can be made thicker as compared with thatof a conventional longitudinal-recording disk, increasing the volume ofmagnetic particles. The advantage of the thermomagnetic recording isthat the resistance to thermal instability rises because, compared withthe magnetic film for magnetic recording, a magnetic film having a verystrong coercive force at room temperature is used as the recording film.

In the case of the thermomagnetic recording, data are usually recordedon a disk by radiating a laser beam to the recording layer to reversethe perpendicular magnetization of the recording film. To reproduce therecorded data, the directions of magnetization are read out directlyfrom the recording film or from the copying layer formed on therecording film by utilizing the optical Kerr effect. This opticalrecording system is capable of forming record bits smaller than thediameter of the beam spot, but has difficulty in reading smaller bitsthan the beam spot in the process of reproduction, presenting theproblem of poor resolution in the range of high linear-recordingdensity. To solve this problem, Japanese Patent Laid-open No. 10-21598,etc. disclose a technique to use, instead of an optical head, a magnetichead for magnetic recording/reproducing devices in the process ofreproduction. In general, the resolution of a reproducing magnetic headis determined by its shield-gap length of the sensor (gap length), andthe gap length of a magneto-resistive head put recently to practical useis reduced to 0.2 μm. The gap length is equivalent to the resolutioncapable of reproducing 0.1 μm record bits. Since the resolution ofcurrent optical disks using a light source of wavelength of 660 nm isabout 0.5 μm, the areal recording density can be raised by five times byusing the above-described head. In reality, however, that largeenhancement of resolution is difficult to achieve. Its main factor isthe shape of the recorded magnetic domains. If data are recorded on adisk with an ordinary optical recording device and recorded opticalspots are examined with a polarization microscope, each spot is in acrescent shape as shown in FIG. 4. Its reason is that the sectionalshape of the recording beam is circular. On the other hand, the gapportion of a magnetic-reproducing head, which absorbs leakage fluxesfrom the medium, is in the shape of a rectangle that is long in thedirection of width of the track. Therefore, the magnetic-reproducinghead cannot efficiently reproduce the magnetic fluxes from thecrescent-shaped recorded magnetic domains. The shorter the bit lengthgets, the lower the efficiency becomes. Thus, even when a magneticreproducing head is used, sufficiently high resolution cannot beachieved.

SUMMARY OF THE INVENTION

The current optical recording and magnetic-head reproducing system hasthe problem that it cannot make full use of the capability of thereproducing magnetic head because the reproduced output decreases as thelength of record bits shortens.

The above problem can be solved by recording data on a diskmagneto-optically so as to make the radius of curvature of the boundaryarcs of each recorded magnetic domain as large as possible and therebymake the domain's shape as rectangular as possible.

The advantage offered by the present invention is mainly that therecorded magnetic domains are rendered near rectangular while data arethermomagnetically recorded on a medium with an optical head; thereforethe efficiency of magnetic reproduction with a GMR head rises. Thus, thecharacteristic of high reproducing resolution of the GMR head can befully utilized. As a result, high-output as well as high-SN-ratioreproduction at high linear recording density can be achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing the dependence of reproduced output on arecorded mark period in the cases when an aperture is provided and whennot provided;

FIG. 2 is a schematic diagram of a magnetic recording device accordingto the present invention;

FIG. 3 is a view showing the construction of a medium for opticalrecording and magnetic reproduction; and

FIG. 4 illustrates recorded magnetic domain shapes written on arecording layer.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 2 shows the outline of an embodiment of a magnetic recording deviceof the present invention. The reference numeral 21 is a laser diode foroptical recording. The numeral 22 is an objective lens to focus a laserbeam on a recording layer 24. A spherical lens is disposed below, or onthe light-outgoing side of, the object lens 22, and an aperture 23 isdisposed further below. The numeral 27 is a magnetic-field modulatingcoil to modulate the directions of magnetization of the recording filmin accordance with signal currents. The numeral 28 is a circuit to drivethe optical laser and the magnetic head. These elements work to recorddata into the recording film as magnetization of upward and downwarddirections. Data are read out from the media by copying themagnetization of the recording layer to the readout layer 25magnetically and reproducing the leakage fluxes from the readout layer25 with a magnetic head 29. The signals read by the magnetic head aredemodulated into the original data by a signal-processing circuit 210.FIG. 3 shows the construction of the recording medium used in thisembodiment. A protective layer 32 of SiN is formed on a polycarbonatesubstrate 31. Formed on the protective layer 32 are a 40 nm-thickrecording layer 33 of Tb₂₁Fe₇₀Co₉, a 70 nm-thick readout layer 34 ofTb₃₅Fe₅₆Co₉, and a 20 nm-thick protective layer 32 of SiN. Used as themagnetic reproducing head was a GMR head that made use of the giantmagneto-resistive effect and of which the track width and the shield gap(Gs) were 1.0 μm and 0.2 μm, respectively. The gap between the head andthe surface of the medium was 0.04 μm. A laser diode was used as thelight source for recording. Its wavelength and spot diameter were 660 nmand 1.0 μm, respectively.

By using the above magnetic recording device and providing an apertureon the laser beam-outgoing side, the effects of the aperture on theshape of recorded spots, the reproduced output, and the resolution wereexamined. The aperture was rectangular, being 0.6 μm long in thedirection of the track and 1.0 μm wide in the direction of width of thetrack. First, by using the aperture and changing the record bit lengthB, “1” was recorded in all bits. Second, for the purpose of comparison,without using the aperture and by changing the bit length, “1” wasrecorded in all bits. FIG. 1 shows the relation between the linearrecording density and the reproduced output. It is clearly shown thatthe reproduced output was maintained at a high level until the bitlength was reduced to about 0.1 μm (0.2 μm in terms of the recorded markperiod) in the first case (11), whereas the reproduced output droppedsharply when and after the bit length was reduced to 0.5 μm (1.0 μm interms of the recorded mark period) in the second case (12).

FIG. 4 illustrates the recorded domain shape observed with apolarization microscope. In the second case, the recorded magneticdomain 41 took a crescent-like shape, its track width Tw being 0.8 μm.The radius of curvature R of its arcs was 0.4 μm. It can be seen fromthis illustration that when the shield gap portion (42; shown by a boldline) of the GMR head comes to directly above the bits while the bitlength B is 0.5 μm (the reproduced output drops sharply in the range ofbit length below 0.5 μm), the GMR head picks up not only the fluxes fromthe record bits but also magnetic fluxes from the adjoining area of theopposite polarity, reducing the reproduced output sharply. In the firstcase, the recorded bit (43) took an almost rectangular shape, reflectingthe shape of the aperture. Its track width Tw was 0.8 μm, and the radiusof curvature R of the arcs constituting its boundary was 3.6 μm. It canbe seen from the illustration that almost whole part of the shield gapportion of the GMR head comes within the boundary of the recorded bitwhile the bit length is 0.5 μm and without an aperture, and hence thehead does not pick up magnetic fluxes from the adjoining area of theopposite polarity. Thus, as compared with those of the second case,higher output and remarkably higher resolution can be achieved in thefirst case. Even in the first case, however, if the bit length isshortened to such an extent (the extent being B=0.1 μm in theembodiment) as the value of “B−A”, “A” being the bulging height of thearcs, becomes less than about ⅔ of the effective gap (Gs/2), Gs beingthe shield gap, the reproduced output is reduced due to the gap-losscharacteristic of the GMR head, regardless of its arrangement not topick up magnetic fluxes from the adjoining area.

Ascertained from the above were two requirements for achieving a highreproduced output and a high-resolution characteristic in anoptical-recording, magnetic-head-reproducing system. First requirementis to render the recorded magnetic domain rectangular as much aspossible by enlarging the radius of curvature of the arcs constitutingits boundary as much as possible. The second requirement is to reducethe gap loss of the GMR head by rendering the bit length aftersubtracting the bulging height of the arcs larger than ⅔ of theeffective gap (Gs/2). More preferably, the bit length after subtractingthe bulging height of the arcs larger than 1 of the effective gap (Gs/2)By putting the two requirements together, the following conditionalexpression can be obtained:

B−A=B−(R−(R ²−(Tw/2)²)^(0.5))≧(⅔)·(G_(s)/2)

When a higher reproduced output is needed, it is desirable to satisfythe following condition:

B−A=B−(R−(R ²−(Tw/2)²)^(0.5))≧G_(s)/2

What is claimed is:
 1. A magnetic recording device comprising a magneticrecording medium having a magnetic recording layer, an optical recordinghead with which data are recorded on the magnetic recording medium and amagneto-resistive head with which data are read out from the magneticrecording medium, said optical recording head being provided with anaperture on the light-outgoing side thereof.
 2. A magnetic recordingdevice according to claim 1, wherein the shape of said aperture isrectangular.
 3. A magnetic recording device according to claim 1,wherein said magneto-resistive head is a giant magneto-resistive head.4. A magnetic recording device according to claim 2, wherein saidmagneto-resistive head is a giant magneto-resistive head.